1. Technical Field of the Invention
This invention generally relates to investment casting, more specifically to lost wax investment casting using wax trees.
2. Description of the Background Art
Lost wax investment casting is a method of creating small molded metal designs. The casting process begins with a wax pattern made from a plastic master mold of the design. The wax pattern has one or more small wax rods, called gates, attached to it in strategically selected areas. The gates are welded onto a wax rod, called a sprue rod, typically at a 45 degree angle. The weld between the gate and the sprue rod is called the filet. The patterns can be welded radially, lengthwise or both. The sprue rod/wax pattern combination is called a wax tree. The bottom end of the sprue rod is inserted into a rubber base, which is then fitted onto the base of a metal flask. The resulting configuration consists of the wax tree being encapsulated by the flask without the patterns touching anything. The flask is filled with slurry, called investment, usually under vacuum pressure. Investment is similar to plaster, becoming hard in a matter of hours. Once the investment has hardened, the rubber base is removed from the flask, and the flask is placed into an oven or other heated space for the dewaxing cycle. During the wax removal, the wax melts and drips out, leaving a negative space where the wax tree used to be. The negative space is an exact duplicate of the wax tree, with the area where the sprue rod and gates were creating a path for molten metal to fill the patterns. The investment mold then cures in the heat, allowing it to withstand the temperatures of the molten metal. Finally, the molten metal is poured into the heated mold, creating a metal version of the wax tree. The investment is removed from the cooled metal, and the patterns can be removed from the tree.
Dozens of problems can arise during the lost wax investment casting process, resulting in casting defects ranging from minor flaws that can be reworked to irreparable defects that require recasting. One common problem results from wax melting within an enclosed space because the adjacent wax has not yet melted, blocking the wax's exit from the investment mold. Wax expands as it melts, causing problems for the investment mold. One of these problems is wax infiltrating pores in the investment mold. This wax can be extremely difficult to remove from the mold and can cause defects in the metal cast.
In addition to casting problems, the traditional wax sprue rod is completely lost in the dewaxing cycle. This is not only wasteful, but also represents a continuing expense that can be eliminated with a non-disposable reusable sprue rod.